We are studying the mechanisms of transcriptional control and the role of primary RNA processing and subsequent decay in the post-transcriptional regulation of gene expression of the major leftward operon of bacteriophage lambda. Transcriptional mapping analysis of the N-mediated RNA (synthesized in the presence of N product) of the leftward operon show that this RNA can be isolated in RNase III-deficient hosts in vivo as a continuous RNA transcript initiated at the leftward promoter, PL. This finding is consistent with the anti-termination model of positive control in which transcription initiated at PL is extended beyond the first transcriptional termination signal (tLl) in the presence of N product. The N-mediated RNA transcript in wild-type(RNase III plus) hosts undergoes a multi-step program of primary RNA processing and decay events involving the following series of steps. (1) As soon as or immediately after the completion of N-mediated RNA synthesis, the PL-proximal RNA segment (PL-N-tL RNA of about 1000 nucleotides in length coding for N product) is very rapidly processed, possibly by RNase III activity. (2) This primary RNA processing leads to very rapid decay of the PL-N-tL RNA. (3) The remaining RNA segment from tL downstream to the 3'-end of the N-mediated RNA transcript is then endonucleolytically cleaved at many sites leading to progressively shorter RNA fragments prior to the final stages of chemical decay. BIBLIOGRAPHIC REFERENCES: Lozeron, H.A., Dahlberg, J.E., and Szybalski, W. (1976): Processing of the Major Leftward mRNA of Coliphage Lambda. Virology 71, 262-277. Lozeron, H.A., Anevski, P.J., and Apirion, D. (1977): Antitermination and Absence of Processing of the Leftward Transcript of Coliphage Lambda in the RNase III-Deficient Host. J. Mol. Biol. 109, 359-365.